1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine.
2. Description of the Related Art
In a well-known internal combustion engine such as a diesel engine, an exhaust gas recirculation path communicating between an exhaust system and an intake system is provided so that the exhaust gas is recirculated into the cylinders whereby the combustion temperature is lowered thereby to reduce the amount of NO.sub.x generated. The larger the amount of the recirculated exhaust gas is, the greater the amount of the generated NO.sub.x can be reduced. It is therefore preferable that an excess air ratio preventing generation of a large amount of smoke or particulates is realized and an exhaust gas is, as much as possible, recirculated in each engine operating condition. The amount of the recirculated exhaust gas is generally expressed by the EGR rate which is the ratio the amount of the recirculated exhaust gas to the amount of the gas (the amount of fresh air and the amount of the recirculated exhaust gas) in the cylinders.
In this way, a combination of the excess air ratio and the EGR rate advantageous for reducing the amount of the generated NO.sub.x and particulates exists for each engine operating condition. Japanese Unexamined Patent Publication No. 2-61347 discloses a method of controlling an EGR control valve arranged in an exhaust gas recirculation passage in such a manner that the current excess air ratio constitutes a target excess air ratio for each engine operating condition.
In the case where the amount of the exhaust gas recirculated is reduced by controlling the EGR control valve, fresh air is liable to flow into the cylinders in a proportionately greater amount. If the amount of the exhaust gas recirculated is increased, in contrast, it becomes more difficult for the fresh air to flow into the cylinders. As a result, this control can realize a combination of the excess air ratio and the EGR rate advantageous for reducing the amount of the generated NO.sub.x and particulates in each engine operating condition.
At the time of transient engine operation, however, in spite of the fact that intake air is delayed, the amount of the recirculated exhaust gas can be reduced excessively in an attempt to realize a target excess air ratio after a change in the engine operating condition, in which case the amount of generated NO.sub.x increases considerably.